Viscosified treatment fluids may be used in a variety of subterranean treatments. Such treatments include, but are not limited to stimulation treatments, sand control treatments and other similar applications. As used herein, the term “treatment”, or “treating”, refers to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. The term “treatment”, or “treating”, does not imply any particular action by the fluid.
Hydraulic fracturing operations generally involve pumping a viscosified treatment fluid (e.g., a fracturing fluid) into a well bore that penetrates a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more cracks, or “fractures”, in the subterranean formation. The fracturing fluid may comprise particulates, often referred to as proppant, that are deposited in the fractures. The proppant function is, inter alia, to prevent the fractures from fully closing upon the release of hydraulic pressure, forming conductive channels through which fluids may flow to the well bore. Once at least one fracture is created and the proppants are substantially in place, the viscosity of the fracturing fluid usually is reduced, and the fracturing fluid may be recovered from the formation.
Treatment fluids are also utilized in sand control treatments, such as gravel packing. In gravel-packing treatments, the viscosified treatment fluid suspends particulates (commonly referred to as “gravel particulates”) for delivery to a desired area in a well bore, e.g., near unconsolidated or weakly consolidated formation zones, to form a gravel pack to enhance sand control. One common type of gravel-packing operation involves placing a sand control screen in the well bore and packing the annulus between the screen and the well bore with the gravel particulates of a specific size designed to prevent the passage of formation sand. The gravel particulates act, inter alia, to prevent the formation particulates from occluding the screen or migrating with the produced hydrocarbons, and the screen acts, inter alia, to prevent the particulates from entering the production tubing. Once the gravel pack is substantially in place, the viscosity of the treatment fluid is often reduced to allow it to be recovered. In some situations, fracturing and gravel-packing treatments are combined into a single treatment (commonly referred to as “frac pack” operations) to provide stimulated production and an annular gravel pack to reduce formation sand production. In some cases, the treatments are completed with a gravel pack screen assembly in place, and the fracturing treatment fluid being pumped through the annular space between the casing and screen. In such a situation, the fracturing treatment usually ends in a screen-out condition, creating an annular gravel pack between the screen and casing. This allows both the fracturing treatment and gravel pack to be placed in a single operation.
Maintaining sufficient viscosity is important in fracturing treatments for particulate transport, as well as to create or enhance fracture width. Particulate transport is also important in sand control treatments, such as gravel packing. Also, maintaining sufficient viscosity may be important to control and/or reduce leak-off into the formation. To provide the desired viscosity, polymeric gelling agents commonly are added to the treatment fluids. Examples of commonly used polymeric gelling agents include, but are not limited to, guar gums and derivatives thereof, cellulose derivatives, biopolymers, and the like. The use of polymeric gelling agents, however, may be problematic. For instance, these polymeric gelling agents may leave an undesirable residue in the subterranean formation after use. As a result, potentially costly remedial operations may be required to clean up the fracture face and proppant pack. Foamed treatment fluids and emulsion-based treatment fluids have been employed to minimize residual damage, but increased expense and complexity often result.
To combat these problems associated with polymeric gelling agents, some surfactants have been used as gelling agents. Certain surfactants, when mixed with an aqueous fluid having a certain ionic strength, are capable of forming a viscous fluid that has certain elastic properties, one of which may be shear thinning. Surfactant molecules (or ions) at specific conditions may form micelles (e.g., worm-shaped micelles, rod-shaped micelles, etc.) in an aqueous fluid. Depending on, among other things, the surfactant concentration, and the ionic strength of the fluid, etc., these micelles may impart increased viscosity to the aqueous fluid, such that the fluid exhibits viscoelastic behavior due, at least in part, to the association of the surfactant molecules contained therein.
As a result, these treatment fluids exhibiting viscoelastic behavior may be used in a variety of subterranean treatments where a viscosified treatment fluid may be useful. Because the micelles may be sensitive to the pH and hydrocarbons, the viscosity of these treatment fluids may be reduced after introduction into the subterranean formation without the need for conventional gel breakers (e.g., oxidizers). This may allow a substantial portion of the treatment fluid to be produced back from the formation without the need for expensive remedial treatments.
Composition and methods from the invention offer an alternative to some types of additives used to increase viscosity of viscoelastic surfactants.